In recent years, with the spread of mobile devices, high-performance secondary batteries including lithium ion batteries have been actively developed. In addition, as stationary electricity storage devices for cars and for home use, hybrid capacitors in which lithium ion secondary batteries and reaction mechanism thereof are applied to an anode have been actively developed. As anode materials in the electricity storage devices, there have been used carbonaceous materials such as natural graphite, artificial graphite and coke which can absorb and desorb lithium ions. However, since these carbonaceous materials allow lithium ions to be inserted into between carbon faces, the theoretical capacity when used as an anode is 372 mAh/g at most. Thus, novel materials which can replace the carbonaceous materials have been actively searched for the purpose of improving capacity.
On the other hand, Si has been drawing attention as a material which can replace the carbonaceous materials. This is because Si can absorb a large amount of lithium to form a compound represented as Li22Si5 and thus can significantly increase the capacity of the anode as compared to the case where the carbonaceous materials are used, so that Si has a potential for increasing the electricity storage capacity of lithium ion secondary batteries or hybrid capacitors.
However, when Si is used singly as the anode material, there was a problem that life of the electricity storage devices is extremely short, because Si phases are micronized by repetition of expansion at the time of alloying with lithium during charge and constriction at the time of dealloying of lithium during discharge, resulting in defects such as detachment of Si phases from an electrode substrate during use and loss of electrical conductivity between the Si phases.
In addition, since Si has an electrical conductivity lower than carbonaceous materials and metal materials and thus limits effective movement of electrons during charging and discharging, Si is used as the anode materials in combination with materials which supplement electrical conductivity, such as carbonaceous materials. Even in this case, however, there is also a problem with charge-discharge performances, particularly with initial charge-discharge and charge-discharge performances at high efficiency.
As a means of solving such problems at the time of using such Si phase as the anode, there are proposed, for example, in JP2001-297757A (Patent Literature 1) and JPH10-312804A (Patent Literature 2), materials or production methods in which at least a part of a lithium-compatible phase, such as Si, is surrounded by an intermetallic compound of Si and a metal as represented by transition metals.
Further, as another solution, there are proposed, for example, in JP2004-228059A (Patent Literature 3) and JP2005-44672A (Patent Literature 4), electrodes or production methods in which an active material phase containing Si phase is coated with a conductive material such as Cu which is not alloyed with lithium.